Charges
Hi all,
from time to time there is a discussion about the construction and the use
of point
charges in molecules. I think there is no doubt within the community, that
for
intermolecular interactions one should use charges ( and point dipoles,
point
quadrupoles, etc ) which are adjusted to reproduce as good as possible the
electrostatic potential of the molecule. For qualitative discussions of
chemical
bonding the use of various population analysis ( Mulliken;
Davidson-Roby-Ahlrichs,
natural orbital populations of Weinhold; Bader; etc. By the way : There is
a very promising new one of Marco Haeser in JACS, Vol. 118, 1996, pp.
7311-7325,
which I think is more rigorous as any previous ones and without any ad hoc
postulates.) are most appropriate. The fitting of the electrostatic
potential of a
molecule is a highly non-trivial subject ( see e.g. Michelle Miller Francl
et al.,
J. Comput. Chem., Vol. 17, 1996, pp. 367-383, and the very recent review by
Sarah L. Price, J. Chem. Soc., Faraday Trans., Vol. 92, 1996, pp. 2997-3008
)
and in my view not a really solved problem, especially for large molecules.
Now I became aware of a series of papers by George G. Hall and his coworkers
about the fitting of electron densities, which I never heart off bevor and
which, I think,
are quite unknown to the community. Hall et al. fit the calculated electron
densities
of molecules to atom-centered gaussians as it is done by many DFT-programs
( "density fitting with an auxillary basis" ) and got a very good ( in
my
view an
excellent ) representation of the electrostatic potential of a molecule.
Those DFT-
programs with charge density fitting are able to calculate the charge
density of
molecules within density functional theory with more than thousand basis
functions
on a single workstation in about an hour of cpu-time ( see e.g. Karin
Eichkorn et al.,
Chem. Phys. Lett. 240 (1995) 283-290 ). As a by-product of the calculation
one gets
an excellent fit of the electrostatic potential of the molecule with
gaussians. After some
minor modifications of the programs one gets from this point charges, point
dipoles,
point quadrupoles, etc. or if one likes it the current bun model of Hall and
coworkes.
My question to the community is now, wether anyone else has similar thoughts
or
even experiences with this sort of density fitted electrostatic potentials ?
Does anyone has an opinion about using such charge models in force field
calculations ( energy minimization of molecular crystals or molecular
dynamics
calculation of fluids ) ? Please send responses to the list, so that anyone
else can
follow the discussion.
Ciao,
Heinz
PS: Here are the references of Hall and coworkers :
George G. Hall and David Martin
Approximate Electron Densities for Atoms and Molecules
Isr. J. Chem. 19 (1980) 255-259
G. G. Hall and C. M. Smith
Fitting Eelectron Densities of Molecules
Int. J. Quantum Chem. 25 (1984) 881-890
G. G. Hall and K. Tsujinaga
The molecular electrostatic potential of some simple molecules
Theor. Chim. Acta 69 (1986) 425-436
K. Tsujinaga and G. G. Hall
The currant bun model of simple molecules
Theor. Chim. Acta 70 (1986) 257-264
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Dr. Heinz Schiffer 65926 Frankfurt am Main
Hoechst AG Phone ++49-69-305-2330
Scientific Computing Fax ++49-69-305-81162
Email : schiffer;at;wia.hoechst-ag.d400.de
schiffer;at;msmwia.hoechst.hoechst-ag.d400.de
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